Frequency stabilization



v Aug. 20, 1946.

' FREQUENCY STABILIZATION Filed Nov. 3, 1942 I 4 REHCM/VCE OSC/ZLATOR 7085 Z0 12\ mmum c) 5 METER 18 A J5 l-+l 14 PILOT 38 40. 42 lf ff "*7 50pm? 00mm 51mm m4. 1 mam I 007M 00mm mum: I 71/5: 45

MR6 ZIEGLER M4NUFL JUHOKOBILSKX INVENTORS.

A 7' mRNEY M. ZIEGLER ETAL 9 Patented Aug. 20, 1946 Marc Ziegler and Manuel Aires, Argentina,

Bank and Trust Com trustee Julio Kobilsky, Buenos assignors to Hartford National pany, Hartford, Conn., as

Application November 3, 1942, Serial No. 464,380

9 Claims. (01. 179-1715) The present invention relates to frequency sta bilization and more p'articularly'to thestabilization at a desired value of the frequency of an oscillator.

In connection with the problem of stabilizing the frequency of an oscillator at a desired value, which is of great importance in radio technique, several solutions have heretofore been proposed. For instance, the use of very selective circuits with great physical constancy, such as crystals, spatial circuits and tuning forks, permits of the construction of oscillators the frequency of which does not vary more than a small fraction of its nominal predetermined value. The frequency of such an oscillator is, however, so well defined by the physical constants of the circuits used that it cannot be shifted, even if desired, from its original value. Only a phase shift can be forced on the oscillation obtained, either by using the properties of electrical circuits or by introducing a time-retarding device. By phase modulation a corresponding frequency modulation may be created at the output, but

the frequency of the oscillator itself does not vary, and the mean frequency of the'output signal remains equal to it.

In many cases,'however, it is not desired to obtain merely a frequency of rigorously known value, but to have an oscillator which shall automatically adjust itself exactly at a frequency which is known only-approximately and which may be varying in time. Again, it may be desired to modulate directly the frequency of an oscillator by periodic variation of one of the circuit constants.

All these cases necessarily require the use of suitable oscillator arrangements the constants of which may readily be modified, so that the frequency may be changed as a function of a controlling magnitude. The control of the circuit may be either mechanical, as by means of variable condensers, moving cores and the like, or electrical, as by means of magnetic saturation,

variation of dielectric constant with temperature,

reactance tubes and the like, and of these methods, the purely electrical one involving reactance tubes is the most generally used nowadays.

The controlling magnitude may be obtained in one of two general ways, namely, either by operating on the basis of the phase difference between the oscillation to be controlled and a pilot oscillation, or by obtaining it from a frequency responsive device which measures the frequency difference between said oscillations.

The first way leads to a synchronization of the two oscillations andto a resultant frequency which is rigorouslyequal to that of the pilot. The classical phasemeasuring methods are limited however to phase differences smaller than 1r/2 which is a given limitation. The second method gives rise merely to a reduction of the frequency, though this reduction can generally be as large as desired.

The ,present invention is more particularly concerned with the frequency reduction methods of stabilization. r

The known systems of frequency control by frequency reduction utilize,-for obtaining the controlling magnitude, the frequency responsive properties of selective circuits. Hence the absolute stability obtainable cannot be better than they absolute stability of the said discriminator circuits, and this suggests the use, for the discriminator, of high quality circuits such as those hereinbefore mentioned. While the use of such circuits is, of course, possible, it will not answer the requirements of linear response to frequency variations over a relatively wide range, which must be satisfied in, for instance, frequency modulation problems. If a wide frequency swing has to be discriminated linearly, only less selective circuits can be used, whereby the inherent instability thereof is introduced into the system. 7

Furthermore, the response of the common discriminator circuits, and, therefore, the resulting controlled frequency, depend either on the amplitude of the applied signal and on the value of a compensating voltage, or on the-correct balance of a symmetric detector arrangement, all of which factors are possible causes of drift from the exact adjustment.

The method of the present invention and the [devices for the realization thereof while having the same basic idea as other frequency stabilizing arrangements, namely, to measure the frequency diiference between the oscillation to be stabilized or controlled and a pilot oscillation and to use the magnitude so obtained for automatically reducing the frequency difference as much as possible, utilizes, in contradistinction to the prior art, a compensated frequency meter the outputof which is zero for a predetermined frequency independently of the battery tension applied.

'By'frequency meter ismeant herein a frequency responsive device commonly known as such, whichincludes only resistances and capaci ties. Accordinglythe present invention dispenses with the use of a symmetric discriminator design comprising selective inductance and capacity cirlong as the variation. is

quency modulation.

aeoasoe cuits and with the use of a stabilized compensat ing tension. 7

Since frequency In ters have already been'discussed in detail elsewhere (see Frequenziometri electronici a d-esviazione,cAlta Frequenza, F. Vecduring anytime, to the number of positive phases of the signal applied to the apparatus.

' meter I6 which produces an output proportional to said frequency difference.

If the RC time of the condenser ClICllll'! is sufficiently small compared withthe period'ofthe.

highest discharged completely, so that the discharges are equal and'the mean value of the current, talgen; 7

over some time including a sufliciently largenumperiods and hence to-the frequency. Moreover, if the amplitude of the signalapplieddoes not influence the loading voltage, the output is in-' dependent of this. amplitude. It can also be shown that, if the frequency of the signal varies,

the output reproduces the variation correctly so slow as compared with frequency to be measured, the condenser :'is each time charged tothe same potential and Preferably the mixer 12 includes a convenient output filter from which the signal of frequency difference resulting from the mixing of the oscillator and pilot outputs is obtained. 7

It will be clear to those skilled-in the art that, with the above arrangement any deviation from the desired frequency due to variation in the constants of the oscillator Ill, as for example a change of capacity with temperature, cannot take place without causing an opposite effect proportional to the frequency change really taking lace as the result of both effects. Analogously, with the well-known .theory of negative feed-back, it

can be shown that the frequency drift of the oscillator when connected to the correcting link is approximately A times smaller than the frequency ber of periods, is proportional to the number-0f drif t t h n A being acenstant of the arrangement equal to th product of the sensitivity lc-of the frequency meter it in voltsper kcJand' the influenced ofthereactance tube 22' in kc. per volt.

Since for successful operation of" the novel method of frequency stabilization the output of the" frequency meter must be properly compenthe longest period of the frequencies passed during therexcursion, and that, even if the frequency modulation has a period of the same order as or even smaller than said longest period, the averagegvalue of the output current, taken over a long time, will still be proportional to the central frequency. I

These general properties of the frequency meter will be utilized in accordance with the present invention for obtaining an improved method and means for frequency stabilization and for obtaining other derivative advantages arising out of the application of'the said improved method and means in particular problems, as will hereinafter be more particularly explained. In the drawing, I i Fig. l is a block diagram illustrating the method offrequency stabilization of the present in' vention, and I I Fig. 2' is a diagram illustrating a frequency modulated transmitter embodying frequency stabilization according to the present invention.

' As already stated, the basic idea underlying the present method is to measure the frequency difference between the oscillation to be stabilized and the oscillation of apilot, and it is therefore obvious that the frequency difference which is to be the direct cause of the controlling action to be applied to the oscillator, must be Within the working range, an univocal function of the oscillator frequency.

Since, physically, the difference frequency'can pass the pilot frequency, taking into-account all' possible factors, including the possibility offre The difference frequency, obtained as shown in Fig. l, by applying the frequency ofan oscillator iil to a mixer l2 simultaneously with theoutput of a pilot i4, is applied to a frequency sated at all times, variations of the elements of this' part of a practical arrangement would be considered as meriting attention, and indeed, investig'a'tion of this 'point'has led to certain valuae ble features of the present invention," as will now bemadeclear. I o

It can beshown that the influence of relative variations da/a and die/k of the constants hand 70 can be made practically negligible as it is equal to their product with the" frequency "difference divided by the factor 'A=alc, which can be made large in practice.

The influence of undesired variations inithe compensating voltage'Vb, when taken separately is, though by no means exaggerated, not at, all

negligible, but the present invention provides means for compensating this effect. As said'be-f fore,'the output current of the frequency'meter fore proportional to the frequency, to. the capacity of the condenser, to thevoltageup to whichthe condenser has been charged and to the resistance in H If now through which the current is now;

the compensating voltage is obtained,

by means of a potentiometer, from the same bat: tery, which provides the tension for loading up the condenser, a change of battery tension will not affect the frequency for which the, compensated output tension ofthe frequency meter is zero.

Such anarr'angement is shown inthe portion of Fig. 2 marked off with chain lines in which a battery 13 provides the operating tension for the.

frequency meter main tube 2 5, and the compensating tension x13 is taken off a potentiometer 23 across said battery 3. The frequency meter, as shown, likewise comprises a condenser C and an outputresistance r, and is preferably provided with an output filtercomprising resistances 28 and .36 and condensers 32 and 34-. o

In the operation of the frequency meter the oscillations present in the output of the mixerstage lZa are applied to the grid of the valve'2 During those half periods of the oscillationior which the ridrispositive'with rfi pecttothe cathode-ofthe valve. the-latter willbeconductive,

and practicallythe full voltage of battery B will be applied to capacitor C through load resistor r and diode I00. During the other half periods of the oscillation, when the grid of valve 24 is negative with respect to the cathode, capacitor C will be discharged through diode Illl, resistor q and the external circuit including battery B, anoderesistor I04 and potentiometer 26.

Resistors I02 and H13, included in the grid circult, of valve 24, are designed to transform the oscillation generated in the output circuit of the mixer stage [2a into a square wave. Across resistor 1' there is generated a discharge capacitor 0 every time the control grid of the valve 24 is positive with respect to the cathode of this valve, so that for each period of the oscillation applied to valve 24, an electrical impulse is obtained in such a form that the mean value of the tension developed across resistor r is proportional to the frequency of the oscillation applied to the control grid of valve 24. The tension so developed across the resistor r is compensated to zero for a given value of the frequency of the oscillation applied to the grid of valve 24 by means of the compensating tension AB so that the resulting voltage assumes positive and negative values in proportion to the sense and amount of deviation of the frequency'of the oscillation from the given value.

The low pass filter following resistor r and constituted by resistors 28 and 30 and condensers 32 and 34 is adapted to suppress the alternating components in the mean voltage developed across the resistor r.

It can be deduced easily, that such an arrangement, if arCB l, this product having the same significance as the factor A=ak above mentioned, that the input frequency of thexfrequency meter forwhich the compensated output is zero, that is just the desired frequency difference between the pilot and the oscillator to be stabilized, is approximately equal to A/rC.

, It follows from this that said desired frequency can be adjusted to a predetermined value by varying either the potentiometer relation A, the output resistance or the capacity of the frequency meter, which is a very interesting feature.

impulse of Y factors, moreover, can each be made adjustable with all the precision required and may present a high degree of constancy. For this reason, and

because the frequency applied to the frequency meter (difference tive variations of be of the order of only kc. or tenths of kc., a high degree of stability of the oscillator frequency when compared with the pilot frequency may be expected, even when working at frequencies as large as mc./s.

It may further be observed that variations of the proper frequency of the oscillator from the output makes it possible and easy to readjust, during operation, the proper cillator to the correct value, when it appears that over a long-time, the frequency shift tends to increase to an inadmissible value.

The above-mentioned relationship between the desired frequency and the fractional compensating voltage, ity of the frequency meter, provides a very confrequency)- by which the relasaid factors are multiplied, may

frequency of the os-,

teristic of the frequency meter 6 justable frequency having all the stability ofinherently non-variable oscillators, suchas quartz oscillators and the like. The desired frequency can be changed, by choosing, for exampleanother position of the potentiometer which determines the fractional compensating voltage. The potentiometer, which may be of the continuously or stepby-step variable type, may be calibrated in kilocycles for adjustment. Alternativelythe output resistance or the .capacitymay be made 31d? justable, and in the latter event, the condenser of the frequency meter would be made variable or comprise a variable portion, as indicated by the arrowin broken lines in Fig. 2. V I I In the foregoing discussion the arrangement and method have been considered for the case of a non-modulated oscillator to be stabilized around a frequencydiifering a fixed amount from that of a non-modulated pilot. It is obvious that, since the response of the frequency meter does not, within a very large range, depend on the amplitude of the signal applied, either the oscillator to be stabilized or the pilot oscillator or both may be amplitude modulated, so long as the amplitude of the signal applied during the most pronounced amplitude minima, does not fall below the lower admissible limit.

If one Or both of the oscillators is frequency modulated, stabilization is also possible, provided that care is takenv that the predetermined frequency difference is sufiiciently greater than the sum of the maximum excursions of the oscillators for the reasons hereinabove set forth.

If care is taken to make the response characlinear between 0 and twice the'desired frequency," and a suitable band pass filter, designed tocut out all frequencies above SGc/s, is placed between the output of the frequency meter and the input of the reactance tubeasindicated at 28 to 54 in Fig.2",the theo retical results obtained can be applied without change to the central frequencies of the free quency modulated signals considered.

The frequency stabilization method of the present. invention may also be applied to frequency modulated transmission, and an embodiment of such application is shown in Fig. 2. Here the transmitteriantenna 36 is connected through a'power output section 38, a driver or doubler 40 and a separator 42 to an oscillator l0a, which is connected to the mixer |2a of the frequency meter unit already described, and to.

a reactance tube 2211, which in the present instance acts both as stabilizer. and. as modulator.

The output of the frequency meter unit, after.

pass filter 28 to 34, is

source of intelligence, such as a microphone (not shown). the frequency meter arrangement maintains the central frequency constant, the output of the oscillator is also'frequency modulated by super-' Thus while I oscillator. Again, itis 1 bility. of the central tically only :upon the cillator used. In general,"the complete stabiliz- I, ingarrangement does n 3 single adjustment the desired frequency,

7 tions of an electrical termined frequency value by means of a circuit 7 and current interrupting merit does :not require more than .-a single quartz possible to make the stafrequency depend pracstability or, the quartz 's.-

ot require more than a .in'orderto adapt it to work at and a change of the value of only one element makes it. possible to stabilize the oscillator around different well-defined fre-' quencies, a feature which may have many "applications in radio communication. The number "of f tubesand parts is relatively small and there is i "n' their assembly. More nothing very critical over, critical elements, such as a tube or a source of tension, may, as 'can'be shown, have variations aslarge 'as 10% without affecting the validity of the results. Finally, stabilizing arrangement does not use any tuned circuits. Hence it can be utilized at any "frequency, and may be assembled and marketed as a universal frequency modulation and stabilization unit. Inclusion of the quartz pilot and of the oscillator gives a frequency modulation transmitting unit adapted to be directly connected to a frequency modulation transmitter.

Although the present invention has hereinabove been described with reference to certain preferredembodiments, We do not wish to be limited thereto as obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

We claim: V 1. The method of detecting frequency deviae oscillation about a prededirect current source it should be noted that the quency of saidjo'scil lation and to :said voltage,

said electrical oscillation connected in series with said source, comprising thesteps of generating electrical impulses for each period of said oscillation having an area substantially proportional to the voltage of said source and a durathe periods of the-highest fretion smaller than quency to be detected, integrating said'impulses to obtain an electrical quantity substantially proportional to the frequency of said oscillation and to said voltage, and simultaneously injecting another electrical quantity derived from said source in, opposition to said first an output quantity which is compensated to zero independently of variations of said voltage at 'a frequency value of the oscillation equal to t'he said predetermined frequency value and the polarity and value of which are proportional.

to the deviation ofsaid electrical .oscillation fromthe predetermined frequency value.

2. The method of detecting frequencyrdevia- I tions of-an electrical oscillation varying about a predetermined mean frequency by'meansof a circuit arrangement comprising a direct current source, a resistance and current interrupting means responsive ,to said electrical oscillation connectedin series with said source and resistance, which comprises the steps of interrupting the current in said circuit in synchronism with half-waves of like sign of saidroscillation to obtain voltage pulsations across said resistance,

transforming said voltage pulsations, into electrical impulses of like sign having an area sub stantially proportional to riods'of the highestfrequency to be.deteeted,-in-

quantity to produce the voltage of said source, anamplitude independent of said oscil-. l-ationand a duration smaller thanthe half-pe and simultaneously inje'c voltage in opposition to sa d quarrt'ity'to produce 7 detecting frequency deviations oscillation varying about apredetermin'ed mean said source, and means said predetermined mean frequency value and the polarity and value ofwhi'ch areproportional to the deviations of said electrical'oscillation from the predetermined frequency value.

3. A compensated frequency discriminator for of an electrical frequency, comprising a circuit containing a source of direct current and a resistance, "means for interrupting the current in said circuit in synchronism'jwith half-waves of like sign of'said oscillation to obtain voltage ipul'sations a'crosssa'id resistance, means for transforming said Voltage pulsations into electrical impulses of like sign having an area substantiallyproportional to the voltage of said source, an amplitude independent of said oscillation and a duration smaller than the half-periods of the highest frequency to be de tected, means to integrate said impulses to obtain a potential substantially proportional to the frequency of said oscillation and to the voltage of to connect a fraction of said voltage in opposition to said pote'ntial'to produce an output potential which is compensated to zero independently of voltag variations in said direct current source at frequency values of the oscillation equal to the said predetermined mean frequency value and the which are prcportionalto the deviations of said electrical oscillation from the predetermined frequ ency value. I

i. A compensated frequency discriminator for detecting frequency deviations of an, electrical oscillation varying about a predetermined mean frequency, comprisingv a thermionic tube having a tov apply said oscillation to said control electrode to produce voltage pulsation across said resistance in synchronism with half-waves of like sign detected comprising a condenser ode electrodes, and a load of said oscillation, means having an. area substantially proportional to the voltage of said source, an' amplitudeindependeht of said oscillation and a duration smaller than the-half-peri-ods of the highest frequencyto be 7 having one electrode coupled to the anode circuit of said tube, two half wave rectifiers having anode and cathresistance, the cathode of one of said rectifiers and the anode of the other rectifier being coupled to electrode of one of said r-ectifiers, and the other electrode of the second of said rectifiers and the other terminal of the load resistance being coupled to an intermediate voltage point in said direct current source, and means to integrate said impulses to produce 'a potential substantially proportional to the frequenc of said oscillation comprising a filter coil- 1 pled to. said load resistance, the voltageat said intermediate point having a value substantially equal and opposite in sign to the value of said potential at a frequency value of the oscillation equal to the said predetermined mean frequency 1 value;

polarity and value of to transform saidL voltagepulsations into electrical impulses of like sign the other electrode of the condenser, one terminal of theload resistance being'coupled to the other 5. A compensated frequency discriminator according to claim 4, in which the condenser is adjustable and varies the frequency at which the said compensated output potential is zero.

6. In a frequency stabilizing system, means for generating a main oscillation the frequency of which is to be maintained at a predetermined value, frequency adjusting means coupled to said generating means, means for generating a pilot oscillation differing from said main oscillation by a predetermined frequency, means for mixing said pilot and said main oscillations to produce a difference oscillation, a circuit including a direct current source and a resistance, means for interrupting the current in said circuit in synchronism with the half-wave of like sign a of said difference oscillation to produce voltage pulsations across said resistance, means for transforming said voltage pulsations into electrical impulses of like sign having an area substantially proportional to the voltage of said source, an amplitude independent on said oscillation and a duration smaller than the half-periods of the highest frequency of said difference oscillation, means to integrate said impulses to obtain a potential substantially proportional to the frequency of said difference oscillation and to said voltage, means to connect said integrating means to said frequency adjusting means to maintain said main oscillation at said predetermined frequency value, and means to inject a fraction of the voltage of the direct current source in opposition to said potential to produce an output voltage compensated to zero independentl of voltage variations in said direct current source at frequency values of the main oscillation equal to said predetermined value.

7. A frequency stabilizing system, according to claim 6, in which the relation between said voltage fraction and said potential is adjustable, and the frequency at which the main oscillation is stabilized is variable before and during operation.

8. In a frequency modulation system, means for generating an electrical oscillation the mean frequency of which is to be stabilized at a desired the frequency of a pilot oscillation generated in said frequency modulation system, frequency adjusting means coupled to said main oscillation generating means, means for mixing said main and said pilot oscillations to obtain a difference oscillation, a circuit including a direct current source and a resistance, means to interrupt the current in said circuit in synchronism with half-waves of like sign of said difference oscillation to produce voltage pulsations across said resistance, means for transforming said voltage pulsations into electrical impulses of like sign having an area substantiall proportional to the voltage of said source, an amplitude independent of said oscillation and a duration smaller than the half-periods of the highest frequency of said difference oscillation, means to integrate said impulses to produce a potential substantially proportional to the frequency of said difference oscillation and to said voltage, means to inject a fraction of the voltage of the direct current source in opposition to the said potential to produce an output voltage compensated to zero independently of voltage variations in said source at frequency values of the main oscillation equal to said desired value, means to connect said integrating means to said frequency adjustin means, and means to vary the amplitude of said output voltage proportional to intelligence to produce frequency modulation of said main oscillation.

9. In a frequency modulation system, a main oscillator the frequency of which may vary about an adjustable stabilized value differing by a predetermined amount from the frequency of a crystal controlled pilot oscillator, means for radiatring the energy of said main oscillation, means for adjusting the frequency of the main oscillator, means for mixing said main and said pilot osoillations to produce a difference oscillation, a thermionic tub-e having a control electrode, an anode and a cathode, a resistance and a direct current source seriall connected between the anode and cathode, a potentiometer connected in shunt with said direct current source, means to appl said difference oscillation to said control electrode to produce voltage pulsations across said resistance in synchronism with half waves of like sign of said difference oscillation, means to transform said voltage pulsations into electrical impulses of like sign having an area substantially proportional to the voltage of said source, an amplitude independent of said difference oscillation and a duration smaller than the half periods of the difference oscillation comprising a condenser having one electrode coupled to the anode circuit of said tube, two half wave rectifiers having anode and cathode electrodes, and a load resistance, the cathode of one of said rectifiers and the anode of the other rectifier being coupled to the other electrode of the condenser, one terminal of the load resistance being coupled to the other electrode of one of said rectifiers, and the other electrode of the second of said rectifiers and the other terminal of the-load resistanc being coupled to the movable arm of said potentiometer, and value differing by a predetermined amount from means to integrate said impulses to produce a po tential substantially proportional to the frequency of said difference oscillation and to the voltage of said source comprising a filter coupled to said load resistance, said movable tap being positioned at a point at which the voltage thereat has a value substantially equal and opposite in sign to the value of said potential at a frequency value of said difference oscillation equal to the mean frequency value of said difference oscillation, said filter circuit being connected to said frequency adjusting means to maintain said main oscillation at the desired stabilized value, and means inserted between said filter circuit and said frequency adjusting means for additionally varying the amplitude of said compensated output voltage proportional to intelligence to produce frequency modulation of said main oscillation about said stabilized value.

MARC ZIEGLER. MANUEL JULIO KOBILSKY. 

